A master cylinder of this type is described, for example, in patent FR-2,724,354.
Devices of this type have been developed very recently for their ability to overcome the dynamic shortcomings of pneumatic brake boosters.
Now it is known that pneumatic brake boosters which are used to provide a force to assist with braking which is added to the actuating force exerted by the driver on the brake pedal and is in theory proportional to this force, have the shortcoming of being able to develop this boost force only after a certain delay from the application of the actuating force.
As the boost force is the result of the difference between the pneumatic pressures which there are on the one hand in a front chamber of the booster, this chamber being connected to a source of partial vacuum, and on the other hand in a rear chamber which is connected to atmosphere during braking, and as the delay in boost force compared with the actuating force is due to a limit on the rate at which atmospheric air is let into the rear chamber through the booster inlet valve at the time of braking, this delay is longer, the more abrupt the braking.
Now, the situations in which braking is rapid are generally emergency situations in which the driver would, by contrast, specifically need the greatest possible boost force as early as possible.
These considerations have quite recently led to the development of master cylinders with hydraulic reaction which, on the one hand, allow the booster inlet valve to open wider and therefore allow an increased air flow rate and, on the other hand, allow a dynamic modulation of the reaction force, that is to say a modulation as a function of the rate of brake application of the fraction of the boost force with which the reaction master cylinder opposes the actuating force in order to adjust the boost force as a function of this actuating force.
Using devices described in documents which have not been pre-published, it is thus possible considerably to reduce the reaction force in the event of emergency braking as compared with the value it would have for normal braking, and this allows a corresponding increase in the braking force available for emergency braking situations.
However, a problem still encountered in developing these devices is the difficulty of cancelling the reaction force in the event of violent braking.